Artifacts map names various storage locations

Rather than specifying an array of URLs for each artifact - the value is now a Map of storage name to artifact URL:

    "artifacts": {
        "notification.hpi": {
            "s3": "https://s3-eu-west-1.amazonaws.com/notification-plugin/jobs/notification-plugin/59/notification.hpi",
            "archive": "http://localhost:8080/job/notification-plugin/59/artifact/target/notification.hpi"
        },
        "notification.jar": {
            "archive": "http://localhost:8080/job/notification-plugin/59/artifact/target/notification.jar"
        }
    }